That's the whole point of 'right to work.' Allow new employees to freeload on the union-negotiated rates for the shop without requiring them to actually join the union. Who would pay if they got the benefits anyway? So the union gets defunded.
Interestingly, back when Jesse Jackson was in the news a lot, he did a lot of advocating for Right-to-Work legislation. The idea was that unions were racially discriminating against blacks, and RtW laws prevented this.